Q: You advised a reader that if he wants to replace his wooden roof with composition shingles he will have to cover the skip sheeting with a plywood deck.

I plan to re-roof my house this summer myself. Right now I have wooden shingles (not shakes) over the skip sheeting, and two layers of comp shingles over that.

My plan was to strip off everything, then apply new comp shingles. I hadn't counted on installing a new deck. The cost of the plywood would be more than the shingles, to say nothing of the extra labor involved.

Can I install comp shingles over skip sheeting? If I must install a new deck, can I use 1/4-inch plywood over the boards? And just what is OSB sheeting?

A: Way to go for tackling the re-roofing yourself. It's a big job, but you can do it. We'll try to answer your concerns and, we hope, allay some of your fears.

You are correct. With three layers of roof covering already on, you should not add a fourth. If you do, you run the risk of the roof becoming too heavy and damaging the roof framing.

Occasionally cedar shingles have been used for roofing. Shingles tend to give a crisp, more pleasing look to certain architectural styles. Tudor-style homes are good candidates for shingle roofs. The difference between shakes and shingles is thickness. Shingles are thinner, taper-sawn and most generally used for side-wall applications. Shakes are thicker, may be sawn or split and are used almost exclusively for roofs.

Composition roofing cannot be applied over skip sheeting. Even new wood shakes or shingles are difficult to install over skip sheeting. Skip sheeting is 1-by-4-inch boards nailed to the rafters with gaps between the boards. The spaces allow the wood shakes or shingles to breathe so that they dry more uniformly. This limits warping and cupping.

When applied properly over skip sheeting, new wood shingles are rigid and allow the roof to be walked on. Composition roofing material never becomes rigid and requires a solid underlayment to stand up to walking.

Do not use 1/4-inch plywood for sheeting. The cost savings is negligible, and it simply is not thick enough to hold the roofing fasteners. New roof sheeting should be 1/2-inch plywood or 7/16-inch OSB.

OSB is alphabet soup for oriented strand board, an engineered lumber product that is widely available at home centers or lumber yards. Like plywood, it comes in 4-by-8-foot sheets. It has become the standard of the industry for roof sheeting, and we recommend it over plywood, mainly because of cost. A sheet of plywood costs about $18, while a sheet of OSB costs about $13.

Either plywood or OSB can be applied directly over the old skip sheeting, but make sure you use a 3-inch nail to nail through both the new sheeting and the skip sheeting and into the rafters.

In terms of cost, 30-year architectural composition roof material runs $30 to $40 per 100 square feet, what roofers call a "square." Figure $40 per square for the sheeting and $40 per square for the roofing for a total of $80 per square and compare it to the cost of a new fire-retardant, wood-shake roof. We have to believe you're ahead with the comp.

A couple of tips: If you're into collecting tools, we suggest you consider investing in a compressor, a framing nailer and roofing stapler for the job. The cost would approach $750, but they would make your life a lot easier and the job go much quicker. Besides, you get to keep the tools for other projects.

Nail guns and compressors are available at rental stores, but depending on the time you take to re-roof, the cost of renting might approach the cost of purchase.

Also, roofing supply houses will "load the roof" with both the OSB and the comp. During tear-off and while placing the sheeting, try to get a buddy or two to help out. Many hands make light work.

And our experience tells us one of the best things in life is a cold one with friends who have just put in a hard day's work.